In Singapore, the air-pollution index surged to a fresh 16-year high as the smoke—blown from fires in Sumatra Island—shrouded the skyline and limited visibility to several hundred meters. Air pollution reached “dangerous” levels in Indonesia’s Riau province, where much of the fires are located.

Air quality also deteriorated in the southern stretches of Peninsular Malaysia, where more districts were classified as having “unhealthy” air quality. The country’s meteorological office warned Wednesday of low visibility in the southern parts of the Strait of Malacca, the body of water separating the peninsula from Sumatra, saying the conditions there were “dangerous” for vessels not equipped with navigational instruments.

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Officials from all three governments have issued repeated health warnings and traded barbs over which country should bear more blame for the haze pollution, a recurring problem in Southeast Asia. With weather analysts predicting several more hazy days ahead, residents are stepping up precautions against the smog, which first hit Singapore and Malaysia over the weekend.

In Singapore, the three-hour Pollutant Standards Index climbed to a high of 172 Wednesday, the highest reading since September 1997 during Southeast Asia’s worst air-pollution crisis. In Malaysia, the Air Pollutant Index rose as high as 160 in Malacca, as of 11 a.m. Wednesday.

Dumai, a city in Riau province, clocked a PSI reading of 460 on Wednesday morning, up from 365 on Tuesday evening, according to Muhammad Rasyid Ridha, chief of the city’s disaster-management agency. “This is very, very bad as it has entered into dangerous levels,” Mr. Ridha said, adding that officials have distributed thousands of face masks to help the local populace cope with the pollution.”

Readings within the 51 to 100 range indicate “moderate” air quality, while 101 to 200 point to “unhealthy” conditions. Measurements between 201 and 300 represent a “very unhealthy” status, while readings above 300 indicate “hazardous.”

Residents in affected areas have heeded government health warnings by cutting back on outdoor activity, drinking more water, and stocking up on medical supplies.

In Singapore, the local Guardian pharmacy chain said its sales of N95-level masks—which have at least 95% efficiency in filtering material the size of sodium chloride particles, grains of salt — have surged amid the hazy conditions, with sales Sunday and Monday coming in at seven times that of the preceding week. The chain also saw demand spike for eye drops, as Monday’s sales jumped by as much as 70% from Sunday.

“I try very hard to be indoors as much as I can, and monitor the [air-quality] levels when I do need to get out,” said Jean Loo, a 28-year-old Singapore-based neuroscience researcher who is two months pregnant with her first child. “I’ve pulled out this week from some activities that require me to walk some distance in the open air, and I try to bring food from home so I don’t need to walk out of office at lunchtime.”

Such woes could continue for several days, partly due to an increase of hotspots—indicating fires—that were detected in Sumatra. According to the Riau station of Indonesia’s meteorological agency, the number of hotspots rose to 187 on Tuesday, including 148 in Riau, compared to Monday’s total of 113.

“Haze could cover the Malacca Strait, because most of the hot spots are at the eastern coast of Riau,” although the extent and severity would depend on weather conditions, said Warih Budi Lestari, a weather analyst at Indonesia’s Meteorology, Climatology and Geophysics Agency. Malaysia’s meteorological office meanwhile warned that low visibility in those waters—currently below five kilometers—could persist until June 26.

Cross-border haze is a recurring problem for Singapore and Malaysia since the 1980s, as prevailing winds blow over smoke from Indonesian forest and peatland fires typically lit as a cheap but illegal way to clear land for plantations. While subsistence farmers and accidental causes have also been blamed, academics say most fires have been detected in plantation areas owned or to be used by palm oil companies.

The region’s worst air-quality crisis occurred in September 1997, when Singapore’s three-hour PSI spiked to a record high of 226. That episode triggered widespread anger in Southeast Asia, and caused billions of dollars in losses from canceled flights, health problems and lost tourism revenue.

It isn’t immediately clear what sort of economic and health impact the latest bout of haze would have. But some tourist spots and small businesses in typically crowded Singapore are already feeling the pinch.

The Singapore Flyer, the city’s landmark giant observation wheel that was recently put up for sale over unpaid debt, had but a handful of patrons riding its capsules Wednesday afternoon. In the business district, hawker centers—Singapore’s quintessential eateries, often without air-conditioning—were also noticeably quieter, with many opting to stay indoors rather than venture out for lunch.

“There are at least 20 to 30% fewer customers at my stall over this week,” said Madam Lim, a food seller at the Amoy Street Food Center in Singapore’s business district. “No one wants to come outside, or sit outside, they keep complaining about the smell. If this continues I will lose a lot of money.”

–I Made Sentana and Joko Hariyanto in Jakarta, Shibani Mahtani in Singapore, and Celine Fernandez in Kuala Lumpur contributed to this article.

About Southeast Asia Real Time

Indonesia Real Time provides analysis and insight into the region, which includes Singapore, Thailand, Indonesia, Vietnam, Malaysia, the Philippines, Myanmar, Cambodia, Laos and Brunei. Contact the editors at SEAsia@wsj.com.

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